This disclosure generally relates to an ammonium/ammonia removal/recovery process, and especially relates to ammonium/ammonia removal/recovery with suppression of struvite formation and/or suppression of sparingly soluble salts precipitation.
Wastewater and/or process water often contain large amounts of ammonium (NH4), which can not be released into the environment. Increasing the pH to form gaseous ammonia (NH3) can result in struvite precipitation and/or the precipitation of other sparingly soluble salts. Struvite is, essentially, soluble magnesium combined with ammonia and phosphate to form magnesium ammonium phosphate (MgNH4PO4.6H2O); a common form of struvite. Struvite is a hard, tenacious solid that scales, adhering to pipe and valve surfaces forming tuberculates. Struvite is extremely difficult to remove from equipment surfaces such as fittings, pipes, and valves. It is generally encountered in sludge lines and downstream processes following biological treatment. Deposits tend to form in pipes where wastewaters stagnate between intermittent flows, but also in areas where there is turbulent flow, pressure change or where cavitations can occur. Struvite deposition occurs most frequently in pipes, pipe elbows, valves, centrifuge weir overflows, pumps, etc.
Struvite deposition is becoming an increasingly widespread problem for a great many wastewater treatment plants. In extreme cases the struvite can reduce the throughput so significantly that the plant operation has to be stopped for the blocked pipe work and other equipment to be cleaned.
Various proposals have been made in order to deal with this problem, many with some degree of success, but none of them have successfully overcome this problem completely. For example, there have been suggestions to prevent struvite formation by removing ammonia and phosphate chemically by the addition of magnesium to raise the pH. Another proposal has been to add ferric salts and anionic polymer. However, the addition of such large amounts of ferric salt could have a detrimental effect on the plant equipment.
In addition to struvite formation at elevated pH, sparingly soluble salts can precipitate with a change of pH. A number of sparingly soluble salts will precipitate and cause clogging and failure of process equipment. Sparingly soluble salts of CaSO4, BaSO4, SrSO4, and Mg (OH)2 are some examples of salts that have a low solubility and can precipitate.
Due to the effects of the release of large amounts of nitrogen in a cleaned wastewater stream, stringent effluent requirements dictate low ammonia/ammonium release, often a total nitrogen release of less than 100 milligrams per liter (mg/L), with as low as less than 3 mg/L sometimes required by regulatory authorities. Hence, there continues to be a need for ammonium/ammonia removal processes that avoid struvite formation and/or precipitation of sparingly soluble salts.